While there may be merit to the argument that tube amplification has superior tone for guitar gear, I don't think that argument applies equally to bass amplification. The real issues with bass amplification are clarity, power, portability and the physics of sound amplification. The physics of bass amplification are different than guitar amplification and you need a lot more power for a bass signal to be audible. Yes we all know you can go direct or mic the amp, but on stage you need a certain amount of power to be heard over everything else.
Tube amplification weighs more than solid state on a wattage basis and the amount of power needed to make a bass signal audible at concert volumes means a bass tube amplifier would be much heavier than a guitar amp to produce the power needed to drive large diameter speakers and horns. So efficiency of signal amplification plays a much larger role in bass amplification than in guitar amplification. In most situations the bass and drums are the bread of the sonic sandwich and guitars, keyboards, vocals are the center. So bass amplification needs to cover frequency ranges at both the top, middle and bottom of the audio spectrum for it to be heard and tube amplification may not have the frequency range or the power needed to be able to cover that spectrum at a reasonable weight, complexity and power consumption.
